The effect of cyclic stress on the enthalpy relaxation behaviour of thermophysically aged unplasticised poly(vinyl chloride)

Unplasticised poly(vinyl chloride) (uPVC) subjected to thermophysical aging and cyclic stress (fatigue aging) was examined by differential scanning calorimetry. The case of thermophysical aging alone was also investigated in order to provide a control. Interestingly, an initial increase in mobility was found at the early stages of aging prior to the usual reduction in mobility for uPVC thermophysically aged without a stress input. It was found in aged uPVC that cyclic stress (at a stress maximum of 40 MPa) caused an initial decrease in the peak temperature for excess enthalpy (TP) followed by an increase, and simultaneously an initial increase in the equilibrium inflection temperature (TEI) followed by a decrease, as a function of number of stress cycles. These results can be attributed to the competing effects of de-aging (rejuvenation) and aging enhanced by the application of cyclic stress. It is concluded that fatigue aging is a combination of two dissimilar structural processes: one permanent and one non-permanent.